1. Field of the Invention
This invention is a continuation-in-part of the pending patent application entitled "Optical Waveguide Display System" filed Apr. 12, 1989 and which has U.S. Pat. Ser. No. 337,141 (still pending). The parent application describes how to switch guided light out of an optical waveguide. Images are formed by systematically switching light out of many parallel waveguides arranged, side-by-side, on a substrate.
2. Prior Art
The prior application details how to switch (tap) guided light out from a waveguide core with sound. Sound is generated by separate transducers oriented alongside the length of a waveguide. Sound waves interact with guided light via the acousto-optic effect. Changes in the waveguide refractive index create light emitting regions at different locations where individual transducers are placed along the length of the waveguide. By using many transducers, and many parallel waveguides, images can be formed on a viewing screen.
Recently, research has revealed a number of limitations inherent in the acousto-optic display method. First, acoustic taps have high electrical drive power requirements because sound must be continuously generated to operate the tap. Typically, sound from a transducer propagates perpindicularly to the direction of light flow through a waveguide core. This sound passes through the waveguide core and is absorbed by material on the other side of the waveguide. Sound traveling beyond the waveguide core does not interact with guided light. Consequently, since most of the acoustic energy travels into non-core regions, sound must continuously be generated by the transducer to operate the tap. This is wasteful, hence the energy requirements of acoustic taps are high.
Second, optical waveguide displays have low resolutions due to the long acousto-optic tap interaction lengths that are needed to efficiently switch light out of the waveguide. The interaction length is the distance light must travel through a waveguide tap before exiting from the core. The screen resolution, as described in the prior application, is determined by the number of consecutive taps which can be placed along the length of a single waveguide. Thus, the longer the tap interaction length, the fewer the number of taps which can be arranged along a given length of waveguide. The acousto-optic tap interaction length is presently estimated to be.apprxeq.10 cm.
The third major disadvantage to acousto-optic taps is the small degree with which sound can change the waveguide refractive index. Sound can induce refractive index changes in silicon dioxide and other commonly used waveguide materials on the order of .DELTA.n.apprxeq.10.sup.-5. Because this refractive index change is extremely small, waveguides must be specially designed so waveguide taps can function with small acousto-optic effects.
Waveguides suitable for use in conjunction with acousto-optic taps are constructed by making the cladding layer surrounding the core very thin. A thin cladding layer allows the evanescent field of core guided light to interact with materials outside the cladding. A small refractive index change in a thin cladding waveguide shifts the evanescent field of the guided light out beyond the cladding. Once outside the cladding, guided light is scattered, or refracted, so it can be seen by a viewer.
Presently, the light guiding ability of a thin cladding waveguide changes when it is exposed to different temperature conditions. A thin cladding waveguide which guides light well at a low temperature will have an increased attenuation at higher temperatures. This increased attenuation is caused by the large refractive index change induced by the thermo-optic effect. For example, a temperature difference in glass of only 20.degree. C. causes a change in .DELTA.n&gt;10.sup.-5. This thermo-optic refractive index change is larger than the highest attainable acousto-optic refractive index change. Consequently, a thin-cladding waveguide used in conjunction with acoustic taps is very sensitive to thermal conditions and can only operate in an environment where the temperature is carefully controlled.
Art related to this invention is disclosed by M. Gottlieb and G.B. Brandt, "Temperature sensing in optical fibers using cladding and jacket loss effects", Applied Optics, Vol. 20, No. 22, Nov. 15, 1981, pp. 3867-3873; M. Gottlieb et. al "Measurement of Temperature with Optical Fibers", ISA Transactions, Vol. 19, No. 4, pp. 55-62; J. R. Hill et. al., "Synthesis and Use of Acrylate Polymers for Non-linear Optics", Organic Materials for Non-linear Optics, Royal Society of Chemistry - Dalton Division, Oxford, 29-30 June 1988, pp. 405-411; J. R. Hill et. al., "Demonstration of the linear electro-optic effect in a thermopoled polymer film", J. Appl. Phys., Vol. 64, No. 5, Sept. 1, 1988, pp. 2749-2751; E.A. Chandross et. al., "Photolocking - A new technique for fabricating optical waveguide circuits", Appl. Phys. Lett., Vol. 24, No. 2, Jan. 15, 1974, pp. 72- 74; Hilmar Franke, "Optical recording of refractive-index patterns in doped poly - (methyl methacrylate) films", Applied Optics, Vol. 23, No. 16, Aug. 15, 1984, pp. 2729-2733; Takashi Kurokawa, "Polymer optical circuits for multimode optical fiber systems", Applied Optics, Vol. 19, No. 18, Sept. 15, 1980, pp. 3124-3129; M. Haruna and J. Koyama, "Thermooptic reflection and switching in glass", Applied Optics, Vol. 21, No. 19, Oct. 1, 1982, pp. 3461-3465; Andrew J. Lovinger, "Ferroelectric Polymers", Science, Vol. 220, No. 4602, June 10, 1983, pp. 1115-1121; D. Bosc and P. Grosso, "Polymer acousto-optic modulator working at 20 Mhz", 2nd International Conference on Passive Components: Materials, Technologies, Processing, Paris, France, Nov. 18-20, 1987, pp. 107-112; D. R. Ulrich, "Overview: Non-linear Optical Organics and Devices", Organic Materials for Non-linear Optics, Royal Society of Chemistry - Dalton Division, Oxford, June 29-30, 1988, pp. 241-263; Brettle et. al., "Polymeric non-linear optical waveguides", SPIE Vol. 824 Advances in Nonlinear Polymers and Inorganic Crystals, Liquid Crystals, and Laser Media (1987 ), pp. 171-177; R. Lytel et. al., "Advances in organic electro-optic devices", SPIE Vol. 824 Advances in Nonlinear Polymers and Inorganic Crystals, Liquid Crystals, and Laser Media (1987), pp. 152-161; NCAP Technology Report, Taliq Corporation, Sunnyvale CA.